1. Field of the Invention
The present invention relates to a flat panel display and, more particularly, to a method of driving an electron-emitting device, a method of driving an electron source formed by arranging a plurality of electron-emitting devices, and a method of driving an image forming apparatus using the electron source.
2. Description of the Related Art
Conventionally, two types of devices, namely thermionic cathode and cold cathode, are known as electron-emitting devices. Known examples of the cold cathodes are surface-conduction emission type electron-emitting devices, field emission type electron-emitting devices (to be referred to as FE type electron-emitting devices hereinafter), and metal/insulator/metal type electron-emitting devices (to be referred to as MIM type electron-emitting devices hereinafter).
A known example of the surface-conduction emission type electron-emitting devices is described in, e.g., M. I. Elinson, "Radio Eng. Electron Phys., 10, 1290 (1965).
The surface-conduction emission type electron-emitting device utilizes the phenomenon that electrons are emitted by a small-area thin film formed on a substrate by flowing a current parallel through the film surface. The surface-conduction emission type electron-emitting device includes electron-emitting devices using an SnO.sub.2 thin film according to Elinson, and in addition a carbon thin film [Hisashi Araki et al., "Vacuum", Vol. 26, No. 1, p. 22 (1983)], and the like.
Further, a surface-conduction emission type electron-emitting device using a coated carbon film is disclosed in Japanese Patent No. 02836015.
Since these devices have a simple structure and can be easily manufactured, many devices can be formed on a wide area.
On the other hand, flat panel displays using liquid crystals have recently replacing CRTs. However, LCD is not of a emissive type, and must be equipped with a backlight. Demands arise for development of emissive type display apparatuses. An example of the emissive type display apparatuses is an image forming apparatus as a display apparatus using a combination of an electron source formed by arranging a large number of surface-conduction type electron-emitting devices and a fluorescent substance which emits visible light upon reception of electrons emitted by the electron source.
An example of the device using a coated carbon film is shown in FIGS. 20A and 20B.
In FIGS. 20A and 20B, reference numeral 1 denotes a substrate; 2 and 3, electrodes; 4, a conductive film; 6, a first gap; 7, a second gap; and 10, a carbon film.
An example of a method of forming the device using a coated carbon film is shown in FIGS. 21A to 21D.
The electrodes 2 and 3 are formed on the substrate 1 (FIG. 21A). The conductive film 4 for connecting the electrodes 2 and 3 is formed (FIG. 21B). A current is flowed through the conductive film 4 to form the first gap 6 at part of the conductive film (this will be called a forming step) (FIG. 21C).
A voltage is applied across the electrodes 2 and 3 in, e.g., an organic substance atmosphere to form the carbon film 10 (FIG. 21D). Simultaneously when the carbon film 10 is formed, the second gap 7 is formed. By this step, the second gap 7 narrower than the first gap 6 can be formed. A portion around the second gap 7 is called an electron-emitting portion 5.
The carbon film contains carbon, and/or a carbon compound.
The step of forming the gap 7 narrower than the first gap 6 formed in the forming step, and improving electron-emitting characteristics will be called an activation step.
If satisfactory electron-emitting characteristics can be obtained by the gap 6 formed in the forming step, the activation step need not always be performed. However, the activation step is preferably performed in terms of the stability of electron-emitting characteristics and the selectivity of the conductive film material.
The device having the gaps formed in these steps preferably undergoes a step called a stabilization step. In this stabilization step, e.g., the device and a device housing are heated to remove/discharge an organic substance present in the device atmosphere so as not to deposit new carbon or carbon compound.
The electron-emitting device formed in the above steps is driven as follows.
That is, a device voltage (Vf) is applied across the electrodes 2 and 3 in a low-pressure atmosphere formed in the stabilization step. At the same time, an anode voltage (Va) is applied to the anode electrode arranged above the device. A current flowing between the electrodes 2 and 3 upon applying the voltage Vf across the electrodes 2 and 3 will be called a device current If, and a current flowing from the device into the anode electrode will be called an emission current Ie.
The device attains a device characteristic (relationship of Ie and If to Vf) as shown in FIG. 6, although the characteristic changes depending on measurement conditions. Note that Vth in FIG. 6 represents a device voltage from which Ie can be monitored.
Japanese Patent Laid-Open No. 8-96700 discloses that the device after the stabilization step maintains the characteristic (memory property) of uniquely determining the emission current Ie and device current If depending on the maximum voltage value (Vmax) of the device voltage Vf applied during the manufacture and driving.
According to this reference, when a device voltage applied at given time exceeds the maximum value (Vmax) of the device voltage Vf applied before, the relationship (device characteristic) of Ie and If to Vf changes. More specifically, the device characteristic characterized by Vmax changes to one characterized by a voltage higher than Vmax (Vmax dependence).
According to this reference, this characteristic (memory property and Vmax dependence) is used to correct variations in device characteristics after the stabilization step.
However, it is difficult to maintain the atmosphere formed in the stabilization step and to form an atmosphere desired in the stabilization step.
For this reason, if the organic substance cannot be satisfactorily removed in the stabilization step, the following problems 1 and 2 may occur.
Problem 1
If the device is driven for a long time after the stabilization step, the device characteristic which should be characterized by the maximum voltage value (Vmax) may vary, i.e., the memory property may be lost. In addition, the device current If and emission current Ie become unstable.
This phenomenon is presumed to arise from structural changes near the gap owing to new carbon or carbon compound deposited near the gap 6 or 7.
As a detailed phenomenon of the problem 1, while the device is driven by a voltage value V1 lower than the maximum voltage value (Vmax), the device characteristic gradually shifts to one (device current characteristic or emission current characteristic with respect to the device voltage) characterized by the voltage value V1. More specifically, the device current If and emission current Ie vs. device voltage Vf curves shown in FIG. 6 shift to the left.
As a result, even if the device is driven by the same device voltage value Vf, the device current If and emission current Ie increase.
This phenomenon becomes more serious in an electron source in which a plurality of devices are connected to a common wiring. The wiring connected to a plurality of devices has a given resistance value, and each device has a characteristic of flowing the device current If. If one device A among the commonly connected devices varies in characteristic (particularly increases If), an effective device voltage applied to an adjacent device B decreases. Considering the phenomenon 1, a decrease in effective device voltage Vf applied to the device B makes characteristic variations in device B larger than characteristic variations in device A. In this manner, when devices are commonly connected, the characteristics of the respective devices successively vary.
Problem 2
If the device is kept driven by the maximum voltage value (Vmax), the device current If and emission current Ie remarkably decrease.
One reason of the phenomenon 2 is presumed to be structural changes near the gap 6 or 7 owing to driving heat or a residual organic substance gas which cannot be removed in the stabilization step.
A method for improving variations in device characteristics supposed to arise from the device atmosphere is disclosed in, e.g., Japanese Patent Laid-Open Nos. 9-50256, 6-289813, 6-289814, and 9-199006, and Japanese Patent No. 2598301.
From the phenomena 1 and 2, the maximum voltage value (Vmax) may-be periodically applied at a predetermined interval during driving at the voltage value V1 so as to maintain the device characteristic characterized by the maximum voltage value (Vmax).
In this case, however, the maximum voltage value (Vmax) maybe unnecessarily applied. For example, in a flat panel display using an electron source formed by arranging many devices, a display video image always changes, and the driving conditions and frequency change between the devices. When the maximum voltage value (Vmax) is periodically applied to all the devices at a predetermined interval, it is undesirably applied to a device which does not vary in characteristic or exhibits only a small degree of variation.
In this case as well as the case 2, the device current If and emission current Ie may greatly change during long-time driving.
By applying the maximum voltage value (Vmax), increases in device current and emission current with respect to a predetermined driving voltage value (Vf) can be suppressed. However, the device characteristic may greatly deteriorate due to an unnecessarily long application time of the maximum voltage value (Vmax) in a device which does not vary in characteristic or exhibits only a small degree of variation.